Piezoelectric ceramics



United States Patent 3,519,567 PIEZOELECTRIC CERAMICS Norio Tsubouchi,Masao Takahashi, Tomeji Ohno, and

Tsuneo Akashi, Tokyo, Japan, assignors to Nippon Electric Company,Limited, Minato-ku, Tokyo-t0, Japan Filed Aug. 7, 1968, Ser. No. 750,794Claims priority, application Japan, Aug. 11, 1967, 42/ 51,505 Int. Cl.C04b 35/00 US. Cl. 252-62.9 1 Claim ABSTRACT OF THE DISCLOSURE Apiezoelectric ceramic is provided consisting essentially of a solidsolution of the system wherein up to 25 atom percent of lead may bereplaced by at least one element selected from the group consisting ofbarium, strontium and calcium.

This invention relates to piezoelectric materials and more particularlyto novel piezoelectric ceramics having excellent properties suitable foruse in particular fields.

One of the typical fields of application of piezoelectric materials isin the manufacture of transducers for trans mitting and receivingultrasonic waves. In this case, the electromechanical coupling factor isthe most essential measure for evaluating in practice the properties ofpiezoelectric materials to be used. The electromechanical couplingfactor is representative of the efiiciency of transforming electricoscillation into mechanical vibration and conversely of transformingmechanical vibration into electrical oscillation, the greater valuethereof standing for better efficiency of interconversion and beingdesired for piezoelectric materials to be used in manufacture oftransducers.

Piezoelectric materials have some other fundamental factors, such asdielectric loss, dielectric constant and mechanical quality factor, allserving for evaluation thereof. As for piezoelectric materials fortransducers, the dielectric loss is desired to be small and thedesirable value of the dielectric constant is large or small dependingon electric loads. The mechanical quality factor which shows thereciprocal proportion of the energy consumed by the material during theenergy conversion is not so much im portant, in this case.

The above matters are described in detail in, for example, D.Berlincourt et al., Transducer Properties of Lead Titanate ZirconateCeramics, IRE Transactions on Ultrasonic Engineering, February 1960, pp.1-6 and R. C. V. Macario, Design Data for Band-Pass Ladder FiltersEmploying Ceramic Resonators, Electronic Engineering, vol. 33, No. 3(1961), pp. 171-177.

It is well known, however, that conventional piezoelectric ceramics, forexample, barium titanate (BaTiO and lead titanate-Zirconate [Pb(Ti-Zr)Oexhibit a small electromechanical coupling factor and are unfit forpractical use. Improvement of this factor has been made only by Way ofincorporating various additional constituents into the ceramics.

The object of this invention is to provide novel piezo- 3,519,567Patented July 7, 1970 electric ceramics which Will exhibit a largeelectromechanical coupling factor.

Another object of this invention is to provide novel piezoelectricceramics suited for use in particular fields, such as in the manufactureof transducers for transmitting and receiving ultrasonic waves.

The elements of ceramic filters and the transducer elements ofmechanical filters provide other important fields of application ofpiezoelectric ceramics. In these fields, it is often required to broadenthe pass band width of the filter. For this purpose, efforts have beenheretofore directed only to development of piezoelectric ceramics havingas high electromechanical coupling factor as possible. Higherelectromechanical coupling factor accounts for smaller capacitance ratioof the ceramic material and hence for broader pass band of the filterwhich is determined approximately in inverse proportion to thecapacitance ratio These efforts, however, obviously have a limit and arecoming to a standstill. It has been found by the inventors that the passband width of the filter can be broadened by employing piezoelectricmaterials having very small mechanical quality factor. This isbecausethe mechanical quality factor (Q of piezoelectric material hassuch a relation with the pass band width (B) of the filter using thatmaterial in accordance with the formula in which the term 1/ Q wasneglected before and because the recent progress of electronics enablescompensation of the loss in energy consumed in the material by otheractive circuit components. Piezoelectric materials whose mechanicalquality factor is made very small are therefore required to provide amechanical or ceramic filter of the broad pass band.

In a resonator made from the piezoelectric material having a reducedmechanical quality factor, frequency response of output amplitudebecomes less sensitive even in the vicinity of the resonant frequency.If the mechanical quality factor is reduced, the piezoelectric materialcould be thus applied to a field where less sensitive or less steepfrequency response is needed. An example of the use of piezoelectricmaterials having a reduce Q is described in K. W. Ragland et al.,Piezoelectric Pressure Transducer with Acoustic Absorbing Rod, TheReview of Scientific Instruments, vol. 38, No. 6 (1967), pp. 740442,wherein lead metaniobate (PbNb O is used.

The well-known piezoelectric ceramic material having small Q value of 5to 10. However, its electromechanical coupling factor decreases at thesame time to below 10%, so that the fields of application are restrictedwithin very special uses. On the other hand, the compositions consistingof lead titanate Zirconate [Pb(Zr-Ti)O and additives such as lanthanumoxide (La O thorium oxide (T110 niobium oxide (Nb O or Wolfram oxide (W0have been known to have a reduced mechanical quality factor whilemaintaining the large electromechanical coupling factor, as disclosed indetail in, for example, Jou'r. Amer. Ceram. Soc. vol. 42, No. 7 (1959),p. 343 and Jour. Amer. Ceram. Soc., vol 48, No. 1 (196 5), p. 54.However, the Q value obtainable from these compositions is at the lowestabout to 100. If it is further reduced, the firing of the materials toform ceramics cannot be completely accomplished or there occurs largedeviation in piezoelectric properties of the ceramics, and hence theproduced ceramics become undesirable for practical use.

It is therefore another object of this invention to provide apiezoelectric ceramic material having the mechanical quality factor of amuch reduced value while leaving the electromechanical coupling factorat a considerable va 118.

A still further object of the invention is to provide piezoelectricceramics for use in ceramic or mechanical filter of a broad pass band.

The ceramic composition of this invention is featured by consistingessentially of a solid solution of Bl(Nl 2Tl1 2) O Bi(Ni Zr quaternarysystem, which contains bismuth (Bi) as a trivalent metallic element,nickel (Ni) and lead (Pb) as divalent metallic elements and alsotitanium (Ti) and zirconium (Zr) as tetravalent metallic elements.

Where the ceramic compositions of the quaternary system (Nl Tl (Ni Zrare represented by the compositional formula [PbTiO ].,[PbZrO wherein t,u, v and w denote a set of molecular ratios and l+u+v+w=l.00

and where a and ,8 are respectively defined by and it has been foundthat the compositions within the range determined by the followingcombinations of a and ,8 have practical utility:

oz: 5 0.01 --z 0.60 0.01 0.10 0.20 0.10 0.30 0.10 0.70 0.50 0.70 1.000.30 1.00 0.20 0.70 0.05 0.70

It will be seen that or also represents (number of Bi atom)/ (sum of Biand Pb atoms) while 5: (number of Ti atom) (sum of Ti and Zr atoms).

In the above compositions, at least one of barium, strontium, andcalcium may be substituted for up to 25 atom percent of lead containedin the original compositions.

The ceramic compositions of this invention show an excellentpiezoelectric activity and hence have a high electromechanical couplingfactor, when within the range defined by the following combinations of aand ,8:

On the other hand, it is possible to reduce the mechanical qualityfactor remarkably with the electromechanical coupling factor being leftat a considerable value, if the ceramic compositions are restrictedwithin the range determined by the following six sets of a and 5:

Excellent piezoelectric properties of the ceramic compositions of thisinvention will be apparent from the following more particulardescription of preferred examples of this invention, as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a diagram of the quaternary system Powdered materials ofbismuth sesquioxide (Bi O nickel monoxide (NiO), lead monoxide (PbO),titanium dioxide (TiO and zirconium dioxide (ZrO were used as startingmaterials to obtain the ceramics of this invention, unless otherwisestated. These powdered materials were weighed to provide final specimenshaving the compositional proportions shown in Table 1. In addition, thepowder of lead monoxide, titanium dioxide and zirconium dioxide wereweighed to obtain the conventional lead titanate zirconate ceramicshaving the compositional proportions shown in Table 2.

The respective powders were mixed in a ball mill with distilled water.The mixed powders were subjected to filtration, dried, crushed, and thenpre-sintered for one hour at a temperature of 750 to 900 C. The sinteredmaterials were again crushed, and after mixing with a small amount ofdistilled water, were press-molded into discs of 20 mm. in diameter at apressure of 700 kg./cm. and sintered in an atmosphere of lead oxide andbismuth oxide for one hour. The sintering was carried out at atemperature of 1250 C. to 1300 C. for the specimens in which a ranged upto 0.20, at 1220 C. for those wherein a ranged up to 0.30, at 1100 C.for those wherein the same ranged up to 0.50, and at 950 C. forspecimens of or exceeding 0.50. The resulting ceramic discs werepolished on both surfaces to the thickness of one millimeter, providedwith silver electrodes on both surfaces, and thereafterpiezoelectrically activated through the polarization treatment for onehour at C. under an applied D.C. electric field of 50 kv./cm. for thosespecimens in which a ranged up to 0.10, while a DC. electric field of 30kv./cm. at 100 C., 40 kv./cm. at room temperature, and 30 kv./ cm. atroom temperature were respectively used in the polarization treatmentfor the specimens in which a ranged up to 0.20, up to 0.30, and over0.30.

After the ceramic discs had been allowed to stand for 24 hours, theelectromechanical coupling factor for the radial mode vibration (k,) andthe mechanical quality factor (Q were measured to evaluate thepiezoelectric activities. The measurement of these piezeoelectricproperties was made according to the IRE standard circuit. The value ofk was calculated by the resonant to antiresonant frequency method. Thedielectric constant (e) and the dielectric loss (tan 6) were alsomeasured at a frequency of 1 kHz.

Tables 1 and 2 show typical results obtained. In the tables, specimensare arranged according to the increase in the value of a and there arealso listed several values of Curie temperature which was determinedthrough measurement of temperature variation in the dielectric constant(e). The novel compositions of the specimens of Table 1 are shown withblack points, while the conventional compositions of the specimens ofTable 2 are indicated by crosses in the same figure.

Comparison of the results for the specimens Nos. 2 and 4 of Table 1 withthose for the specimen No. 4 of Table 2 will reveal that the greatest kvalues of the novel ceramics of this invention are far superior to themaximum k value of the conventional lead titanate-zirconate ceramicswhich has been known as the most excellent piezoelectric ceramicmaterial. Moreover, comparison of the results for the specimens Nos. 1to 18 in which a is 0.01 to 0.20 of Table 1 with those for the specimensof Table 2, particularly between the novel and conventional ceramicswhere the 3 values are same or similar to each other, indicates that theceramics of this invention have a remarkably improved k value. Thislatter fact will be more clearly understood from FIG. 2, wherein thecurve of a thick line represents the k, values of the novel ceramicswith the a value being fixed at 0.05 and the 3 value being varied, whilethe curve of a fine line shows the k values of conventional leadtitanate zirconate ceramics with the varying {3 value.

As is seen from the above, this invention provides excellent, usefulpiezoelectric ceramics having superior piezoelectric activity. In thenovel ceramics of this invention, the superior piezoelectric activity asmentioned above are available when the compositions fall within thepolygonal area A-BC-H-I of FIG. 1. The sets of the or and ,8 values ofthe vertices of this polygon ABCHI are as follows:

Where the on value is less than or more than that falling within theabove-mentioned area, the value of k becomes fairly or very small. Incase the p value does not fall within said area, there results unusefulpiezoelectric ceramics having markedly inferior piezoelectric activity.

Table 1, particularly Nos. 16 to 38, show that the Q value is remarkablyreduced in the ceramics of this invention, while the k value ismaintained at a fairly high value or in some cases increased as will beseen from the comparison with Table 2. Thus, this invention provides theuseful piezoelectric ceramics having peculiar properties favorable foruse in particular fields such as the element of broad band widthfilters. These peculiar properties, namely very small Q values with kleft at considerable values, are obtainable where the ceramiccompositions fall within the range as defined by the polygon C-D-EFGH ofFIG. 1. The combinations of the a and 18 values at the vertices of thepolygon are as follows:

999. 95 meeqqwm 000000 92"."999 \IOOUn-H- 000000 122 If a is smallerthan that falling within this range, Q increases. Where or exceeds thisrange, accomplishment of the sintering in manufacture of ceramicsbecomes difficult and besides k so decreases as to make the practical 6use impossible. In case B is smaller than the said range, k extremelydecreases as well.

In view of the above, it is determined that the ceramics of thisinvention, if required to apply to a practical use, should have thecompositions falling within the area A-B-D-E-F-G-H-I of FIG. 1. Theceramics of this effective compositions show excellent piezoelectricproperties and have a high Curie temperature, as shown in Table 1, sothat the piezoelectric activity may not be lost up to elevatedtemperature.

The quaternary system of of this invention exists in a solid solution ingreater parts of compositions and such a solid solution has aperovskite-type crystalline structure. FIG. 3 shows the crystallinephases of the ceramic compositions falling within the area ABC-DEFGHI ofFIG. 1 as determined at room temperature by the powder method of X-rayanalysis. These compositions have a perovskite-type crystallinestructure and belong to either the tetragonal phase (indicated by T inthe figure) or the rhombohedral phase (indicated by R). The morphotropicphase boundary'is shown with a thick line in the figure. In general, thevalue of k is remarkably great in the vicinity of this phase boundary.

It will be apparent that the starting materials to be used inmanufacture of the ceramics of this invention are not limited to thoseused in the above examples. In detail those oxides may be used insteadof any starting material of the above examples, which are easilydecomposed at elevated temperature to form required compositions, asexemplified by Pb O for PbO in the examples Nos. 6, 14, 23, 27 and 31.Also, those salts such as oxalates or carbonates may be used instead ofthe oxides used in the examples, which are easily decomposed into therespective oxides at elevated temperature. Otherwise, hydroxides of thesame character as above may be used instead of the oxides. Moreover,excellent piezoelectric ceramics having similar properties to the aboveexamples are still also obtainable by preparing separately powderedmaterial of each of Bi(Ni Ti )O Bi(Ni Zr )O PbTiO and PbZrO in advanceand by using them as starting materials to be mixed subsequently.

The example No. 8 of Table I reveals that the excellent piezoelectricactivity is obtained by replacing a part of lead by strontium. Ingeneral, the piezoelectric activity of the compositions of the typewhere lead titanate or zirconate is contained is not lost even when upto 25 atom percent of lead contained in the composition are replaced byat least one of barium, strontium and calcium. This fact can be presumedfrom a number of researches, for example, the U.S. Pat. 2,906,710. Thus,the substitution as mentioned is permissible in the ceramic compositionsof this invention.

It is usual that zirconium dioxide (ZrO available in the market containsseveral percent of hafnium dioxide (HfO Accordingly, the ceramiccompositions of this invention are allowed to contain small amounts ofsuch oxides or elements as existing in the materials available in themarket. Moreover, it is presumable that addition of a small amount ofsome additional agent to the ceramic compositions of this invention mayfurther improve the piezoelectric properties, from the similar factrecognized in the conventional lead titanate zirconate ceramics. It willbe understood from the foregoing that the ceramic compositions of thisinvention may include appropriate additives.

While there have been described what at present are I believed to be thepreferred examples of this invention, it will be obvious that variousmodifications can be made therein without departing from the scope ofthis invention and that this invention covers all the ceramiccomposilons as specified in the appended claims.

TABLE 1 Composition Curie kn Tan 5, temp a ,6 percent Qm e percent C.)

0.01 0.60 15 300 410 1. 3 0. 01 0. 48 61 280 950 1. 5 0. 01 0. 10 12 910180 2. 0. 02 0. 48 65 300 880 1. 6 0. 0. 70 7 70 300 1. 4 0. 05 0.55 43180 780 1. 7 0.05 0. 48 55 i 270 540 1. 6 0. 05 0. 48 64 320 920 1. 6 0.05 0. 40 40 330 560 3. 2 0. 05 0. 25 24 490 320 3. 2 0. O5 0. 10 13 470290 3. 3 0.10 0.55 34 250 1, 070 1. 6 0. 10 0. 48 48 350 500 1. 6 0. 100.35 31 380 370 2. 5 0. 10 0. 18 330 290 2. 6 0. 20 0.70 27 160 220 2. 10. 20 0. 48 37 120 470 1. 7 0.20 0. 10 8 180 210 4. 1 0.30 1.00 9 42 2704. 3 0. 30 0.90 9 23 330 3. 3 0. 30 0. 70 29 29 870 3. 4 0. 30 0.60 321, 450 5. 0 0. 0. 48 28 28 1, 000 3. 9 0. 30 0. 30 16 710 5. 9 0. 30 0.1O 6 70 265 3. 8 0. 40 0. 80 37 38 1, 310 3. 8 0. 40 0.60 24 24 1, 2908. 9 0. 40 0. 40 11 40 630 5. 3 0. 40 0. 20 9 56 410 5. 7 0.50 1. 00 1839 820 4. 6 0.50 0. 90 15 10 295 4. 4 0. 0. 75 17 24 500 5. 9 0. 50 0.13 30 1,050 9. 2 0. 50 0. 48 9 40 940 4. 6 0.60 O. 80 17 7 305 9. 2 0.701.00 8 27 550 5. 9 0. 0. 70 11 22 480 6. 8 0. 70 0.50 7 40 620 9. 2

NOTE.II1 manufacture of the specimens whose Nos. have a single asterisktriplumbic tetroxide (PbsOi) was used instead of lead monoxide (PbO) asone of the starting materials. In manufacture of the specimen withdouble asterisk strontium carbonate (SrCOa) calculated on the basis ofstrontium monoxide (SrO) was used to replace 5 atom percent 01 lead (Pb)by strontium (Sr).

Nora-For the specimens Nos. 1 and 2, evaluation of piezoelectricactivity was impossible.

.What is claimed is:

1. Piezoelectric ceramics consisting essentially of the compositionwhich is represented by the formula v [P132103] w where t, u, v and wdenote a set of molecular ratios and t+u+v+w=1.00, and where up to 25atom percent of lead may be replaced by at least one element selectedfrom the group consisting of barium, strontium and calcium, and whichfalls Within the polygonal area A-B- DEF-GHI of FIG. 1 of the drawings,the vertices of said polygonal area being determined by the followingcombinations of on and {3, where said a and B are respectively given bya=t/(t+v)=u/(u+w) and O. 01 0 60 0. 01 0 10 O. 30 0. 10 0. 7O 0 50 0.70 1. O0 0. 30 1. 0D 0. 20 0. 70 O. 05 0. 70

References Cited UNITED STATES PATENTS 3,068,177 12/1962 Sugden 252-62.93,268,783 8/ 1966 Saburi.

FOREIGN PATENTS 888,740 2/1962 Great Britain.

TOBIAS E. LEVOW, Primary Examiner I. COOPER, Assistant Examiner U.S. Cl.X.R. 106-39

